1. Field of the Invention
The present invention relates to an ultrasonic transducer for transmitting and receiving ultrasonic waves, and a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter using the ultrasonic transducer.
2. Description of Prior Art
In recent years, an ultrasonic flowmeter which measures the time of flight of an ultrasonic wave across the propagation path, and determines the passing speed of a fluid, thereby to measure the flow rate has come into use for a gas meter or the like.
FIG. 1 is a diagram showing the measurement principle of an ultrasonic flowmeter. As shown in FIG. 1, a fluid flows in the direction indicated by an arrow at a velocity V in a tube. A pair of ultrasonic transducers 101 and 102 are oppositely mounted on a tube wall 103. The ultrasonic transducers 101 and 102 are respectively configured with piezoelectric vibrators such as piezoelectric ceramics as electrical energy/mechanical energy conversion elements. Herein, the ultrasonic transducer 101 is used as an ultrasonic transmitter, and the ultrasonic transducer 102 is used as an ultrasonic receiver.
The operation is as follows. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 101 to the piezoelectric vibrator, the ultrasonic transducer 101 emits an ultrasonic wave into an external fluid along a propagation path denoted by L1 in the diagram. Then, the ultrasonic transducer 102 receives the propagated ultrasonic wave, and converts it into a voltage. Subsequently, the ultrasonic transducer 102 is used as an ultrasonic transmitter, and the ultrasonic transducer 101 is used as an ultrasonic receiver. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 102 to the piezoelectric vibrator, the ultrasonic transducer 102 emits an ultrasonic wave into the external fluid along a propagation path denoted by L2 in the diagram. Then, the ultrasonic transducer 101 receives the propagated ultrasonic wave, and converts it into a voltage.
Further, with such an ultrasonic transducer, if an alternating voltage is successively applied thereto, ultrasonic waves are successively emitted from the ultrasonic transducer. Accordingly, it becomes difficult to determine the time of flight. For this reason, in general, a burst voltage signal using a pulse signal as a carrier wave is used as a driving voltage. Hereinafter, the measurement principle will be described in more details. Upon application of a burst voltage signal for driving to the ultrasonic transducer 101, an ultrasonic pulse wave is emitted from the ultrasonic transducer 101. The ultrasonic pulse wave propagates through the propagation path L1 with a length L, and reaches the ultrasonic transducer 102 after (time of flight) t hours. With the ultrasonic transducer 102, the propagated ultrasonic pulse wave can be converted into an electrical pulse wave at a high S/N ratio. By using the electrical pulse wave as a trigger signal, the ultrasonic transducer 101 is driven again to emit an ultrasonic pulse wave. This device is referred to as a sing-around device. The time required for an ultrasonic pulse to be emitted from the ultrasonic transducer 101, and propagate through the propagation path to reach the ultrasonic transducer 102 is referred to as a sing-around period. The inverse thereof is referred to as a sing-around frequency.
In FIG. 1, a reference character V denotes the flow velocity of the fluid flowing in the pipe, C denotes the velocity of an ultrasonic wave in the fluid, and xcex8 denotes the angle between the direction of flow of the fluid and the direction of propagation of an ultrasonic pulse. When the ultrasonic transducer 101 is used as a transmitter, and the ultrasonic transducer 102 is used as a receiver, the following equation (1) holds:
f1=1/t1=(C+V cos xcex8)/Lxe2x80x83xe2x80x83(1)
where t1 denotes the sing-around period which is the time for an ultrasonic pulse emitted from the ultrasonic transducer 101 to reach the ultrasonic transducer 102, and f1 denotes the sing-around frequency.
In contrast, when the ultrasonic transducer 102 is used as a transmitter, and the ultrasonic transducer 101 is used as a receiver, the following equation (2) holds:
f2=1/t2=(Cxe2x88x92V cos xcex8)/Lxe2x80x83xe2x80x83(2)
where t2 denotes the sing-around period therefor, and f2 denotes the sing-around frequency.
Accordingly, the frequency difference xcex94f between both the sing-around frequencies is expressed as the following equation (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path for the ultrasonic wave, and the frequency difference xcex94f:
xcex94f=f1xe2x88x92f2=2V cos xcex8/Lxe2x80x83xe2x80x83(3)
Namely, it is possible to determine the flow velocity V of the fluid from the length L of the propagation path for the ultrasonic wave, and the frequency difference xcex94f. Therefore, it is possible to determine the flow rate from the flow velocity V.
Such an ultrasonic flowmeter is required to have a high degree of precision. In order to improve the precision, the acoustic impedance of a matching layer becomes important which is formed on the transmitting and receiving surface of ultrasonic waves in the piezoelectric vibrator configuring the ultrasonic transducer for transmitting ultrasonic waves to a gas, or receiving the ultrasonic waves propagated through the gas. The acoustic impedance of the piezoelectric vibrator for generating the ultrasonic vibrations is about 30xc3x97106. The acoustic impedance of air is about 400. The ideal value of the acoustic impedance of the acoustic matching layer is about 0.11xc3x97106. Further, the acoustic impedance is defined as the following equation (4):
Acoustic impedance=(density)xc3x97(sound velocity)
Therefore, a low density material, such as a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, is used for the acoustic matching layer for controlling the acoustic impedance at a low level. Alternatively, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like. The method is disclosed in Japanese Patent Publication No. 2559144, or the like.
For the acoustic matching layer used in a conventional ultrasonic transducer used for an ultrasonic flowmeter, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like, as described above. For this reason, there occur the following problems. The medium tends to be heterogeneous due to fracture of the glass ball under pressure, separation under insufficient pressure, foaming of the peeled molten material, or the like. Accordingly, variations occur in characteristics, which then generates variations in device precision. Further, there also occur the following problems. For example, since the acoustic matching layer is exposed to a gas, the surface is collapsed by the moisture, or the layer is easily deteriorated by a chemically active substance, resulting in inferior durability.
The present invention has been completed for solving such problems. It is an object of the present invention to provide a high sensitivity ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter.
An ultrasonic transducer of the present invention is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. With this configuration, it is possible to obtain the ultrasonic transducer having an acoustic matching layer which has a low acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, the ultrasonic transducer shows a narrow range of characteristic variations due to the high homogeneity of the dry gel.
Further, if the ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers.
First of all, the ultrasonic transducer is so configured that the piezoelectric element and the acoustic matching layer are chemically bonded with each other.
Secondly, the ultrasonic transducer is so configured that the piezoelectric element is mounted on the inner side of a hermetically sealed case, and the acoustic matching layer is mounted on the outer side of the hermetically sealed case opposed to the mounting position of the piezoelectric element.
Thirdly, the ultrasonic transducer is so configured that the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic oscillation frequency at the position of the outer side opposed to the mounting position of the piezoelectric element, and the acoustic matching layer mounting part is filled with the dry gel of an inorganic oxide or an organic polymer.
Fourthly, the ultrasonic transducer is so configured that the hermetically sealed case and the acoustic matching layer are chemically bonded with each other.
Fifthly, the ultrasonic transducer is so configured that the hermetically sealed case is made of a conductive material.
Sixthly, the ultrasonic transducer is so configured that the conductive material is a metal material.
Seventhly, the ultrasonic transducer is so configured that the dry gel constituting the acoustic matching layer has a density of 500 kg/m3 or less, and a mean pore diameter of 100 nm or less.
Eighthly, the ultrasonic transducer is so configured that the solid skeletal part of the dry gel contains at least silicon oxide or aluminium oxide as a component.
Ninthly, the ultrasonic transducer is so configured that a protective layer with a density of 800 kg/m3 or more, and a thickness of 10 xcexcm or less is formed on the surface of the acoustic matching layer.
Tenthly, the ultrasonic transducer is so configured that the protective layer is made of any of a metal material, an inorganic material, and a polymer material.
Eleventhly, the ultrasonic transducer is so configured that the protective layer is made of any of aluminium, silicon oxide, aluminium oxide, amorphous carbon, and polyparaxylene.
In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of brazing (or soldering) the dry gel to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance.
In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of forming the acoustic matching layer. The acoustic matching layer formation process includes: a deposition step of applying a gel raw material solution to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted; a solidification step of obtaining a wet gel from the gel raw material solution; and a drying step of removing a solvent in the wet gel to obtain a dry gel. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance.
Further, if the method for manufacturing an ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers.
First of all, in the ultrasonic transducer including the piezoelectric element mounted on the inner side of a hermetically sealed case, the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic wave length at the position of the outer side opposed to the mounting position of the piezoelectric element of the hermetically sealed case. In the method for manufacturing the ultrasonic transducer, the gel raw material solution is applied to the acoustic matching layer mounting part.
Secondly, in the method for manufacturing the ultrasonic transducer, a protective layer is formed on the surface of the acoustic matching layer by a dry deposition method.
Further, an ultrasonic flowmeter of the present invention includes: a flow rate measuring part through which a fluid to be measured flows; a pair of ultrasonic transducers for transmitting and receiving an ultrasonic wave mounted at the flow rate measuring part; a measuring circuit for measuring the time of flight of an ultrasonic wave between the ultrasonic transducers; and a flow rate operation means for calculating the flow rate based on the signal from the measuring circuit, each of the ultrasonic transducers being made up of a hermetically sealed case by which the fluid to be measured and the piezoelectric element are shielded from each other. With this ultrasonic flowmeter, it is possible to achieve the improvement of the stability of the flow rate measurement due to the high sensitivity and the narrow range of variations in characteristics of the ultrasonic transducers.
As described above, the present invention provides such a configuration that the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. Accordingly, it is possible to obtain an ultrasonic transducer having an acoustic matching layer which is very lightweight and has a small acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, it is also possible to obtain the ultrasonic transducer which shows a narrow range of characteristic variations, and is stable due to the high homogeneity of the dry gel. Still further, upon formation of the dry gel of an inorganic oxide or an organic polymer, the OH group on the piezoelectric element surface or the case surface and the component of the raw material react and chemically bonded with each other to ensure the bond therebetween. Therefore, such an excellent effect can also be expected that an adhesion layer-free, or a so-called adhesion layer-less ultrasonic transducer is obtainable.
Such objects and advantages of the present invention will become more apparent from the following description of embodiments given by reference to the accompanying drawings.